The present invention relates to a sensor apparatus for sensing a condition inside a barrier, such as a ship hull or an oil pipeline. In particular, the invention relates to a sensor apparatus which communicates inductively with a passive sensor located inside the barrier. The sensor apparatus comprises no active electrical components inside the barrier and does not require that external power or data lines penetrate the barrier in order to power or communicate with the sensor. In addition, the present invention includes a number of methods for nulling out the eddy currents which are generated in a conductive barrier during operation of the sensor apparatus to thereby enable the sensor apparatus to work effectively through metal or other conductive barriers.
Sensors have been used for decades to sense environmental conditions inside structures. In the hydrocarbon production industry, for example, sensors are commonly used to sense the pressure and temperature of the production fluid flowing through pipelines. Many prior art sensors typically require both a power source and a data link in order to communicate with an associated data processing apparatus. Even if the sensor includes its own power source, such as a battery, a data line is still required to provide the necessary data link to the sensor. Therefore, the structure in which the sensor is located must normally be breached in order to accommodate the data line. However, breaching the structure for this purpose is undesirable, especially in structures containing high pressure fluids, such as oil pipelines.
Although inductive power and data communications systems exist which can wirelessly transmit both power and data through many materials, including those typically used to make oil pipelines, these systems normally require active electrical components both inside and outside of the structure. However, the use of active electrical components in certain environments is problematic. For example, with the exception of certain silicon carbide electronics, which are usable at temperatures of up to around 250° C. but are not available for most applications, even the highest rated Mil-Spec electrical components typically begin to fail at around 125° C. In this regard, even the high temperature solders which are used in such components usually begin to melt at about 220° C. Consequently, in certain structures, such as subsea oil pipelines, where temperatures can reach 180° C. to 320° C. or higher, the use of active electrical components is not practical.
Furthermore, although inductively interrogated passive sensor systems exist in the prior art, to the inventors' knowledge these systems do not work when the passive sensor is positioned entirely within or against a conductive barrier.
For example, U.S. Pat. No. 7,159,774 describes an inductively interrogated passive sensor system in which a passive magnetic field response sensor is excited by a radio frequency (RF) signal and in response thereto generates a magnetic field which is indicative of a condition to be sensed. The passive magnetic field response sensor comprises either an inductor-capacitor (L-C) circuit or an inductor-resistor-capacitor (L-R-C) circuit, and the RF signal is generated by an antenna which is positioned on the opposite side of a barrier from the sensor. When excited by the RF signal, the L-C or L-R-C circuit generates a magnetic field which is dependent on the values of the inductance, capacitance and/or resistance of its components, which in turn are dependent on the condition or conditions to which the components are exposed, and the inductor transmits this magnetic field back to the antenna. However, this system will not work when the antenna and the entire passive sensor are positioned on opposite sides of a conductive barrier. Instead, the inductor must be positioned on the same side of the barrier as the antenna and connected to the remainder of the L-C or L-R-C circuit through a hole in the barrier. In addition, even when positioned outside the conductive barrier, the system will not work when the inductor is positioned against the barrier due to the large eddy currents which the magnetic field generates in the barrier.